1. Field of the Invention
The present invention relates to a digital video effects generators, and in particular, to read-side digital video effects generators.
2. Description of the Related Art
Digital video effects systems, including digital video effect generators and video combiners, are well known and widely used in the art. Generally, a digital video effects system receives at least two video input signals, with one selected for use as a video fill signal and the other selected for use as a video background signal. This selection involves another input signal, i.e. a key signal, which selectively keys the fill video over the background video. If the video input signals are digital, they are typically in accordance with the SMPTE D1 standard. However, sometimes one or more of the input signals may be in accordance with the SMPTE D2 standard, thereby requiring conversion to the SMPTE D1 standard. These video signals can then be processed in accordance with a number of well known video effects. For example, the fill video can be keyed onto the background video with perspective or rotational effects, or a video image can be constructed which appears to be a three-dimensional object with fill video occupying the faces of the object. While all of the foregoing, and much more, can be achieved with conventional digital video effects systems, many limitations have existed.
One problem in conventional digital video effects systems involves the sampling of analog video signals to produce the digital video input signals. A conventional system often uses a sampling frequency of approximately 13.5 MHz in accordance with the SMPTE D1 standard. With the luminance bandwidth of an NTSC compatible component video signal being approximately 5.5 MHz, substantial signal sampling losses often result. Although the Nyquist sampling criterion is fulfilled by virtue of the sampling frequency being at least twice as high as the luminance bandwidth, the higher frequency components of the luminance signal nonetheless experience significant aperture loss, due to the (sin x)/x frequency response of the sampled video signal.
Further, and worse, losses are incurred with a conventional digital video effects system where spatial transformations are performed using linear interpolation. For example, generating subpixel offsets has the effect of lowering the apparent sampling frequency, thereby effectively placing the frequency responses of the sampled signals even further out on the (sin x)/x frequency response curve.
Another problem with conventional systems involves video motion detection. Some systems use video motion detectors for determining whether the subject matter of a video image has moved between temporally adjacent fields. Current advanced video motion detectors use a threshold detector, which means that compensation for such motion is binary. In other words, either full motion compensation or no compensation is provided to the video information. This results in objectionable and perceptible mode changes.
A problem related to the foregoing involves the use of keyed transformations of an incoming video signal. Non-spatial transformations (e.g. blur or mosaic) are typically done according to a key signal in a binary manner, i.e. the incoming video information is either transformed fully or not at all according to whether and when the key signal exceeds a threshold. This also often results in undesirably distorted images.
Another problem with conventional systems involves their use for spatial transformations (e.g. constructing three-dimensional images). Conventional generators use only post-transform keyers. Output keying of the fill and background video signals is not done until after transformation of the video. Thus, whenever keyed transformations (e.g. spatial or non-spatial) are to be used, multiple edit passes are required, i.e. one for each transformation.